This invention relates to combustion engines such as those used to propel motor vehicles, and particularly to such vehicles with multi-fuel capability.
Motor vehicle engines of the prior art have been adapted to operate with a variety of fuels such as gasoline or a constant mixture of gasoline and methanol. However, since each particular fuel or constant fuel mixture has a different energy content and therefore requires a different air/fuel (A/F) mixture for optimal combustion, an engine designed for one fuel or constant fuel mixture did not operate optimally, if it operated at all, with a different fuel or constant fuel mixture.
Therefore the prior art shows systems in which, for some combinations of fuels, the relative concentrations of the fuels in the mixture can be sensed and the A/F ratio provided to the engine controlled in response thereto for optimal combustion. These systems have been designed for mixtures of gasoline and methanol and include optical, microwave and capacitive sensors for methanol concentration.
An optical sensor is shown in U.S. Pat. No. 4,438,749 to Schwippert, issued Mar. 27, 1984, and the paper "Vehicle Operation with Variable Methanol/Gasoline Mixtures", published in May 1984 as part of the VI International Symposium on Alcohol Fuels Technology in Ottawa, Canada. This sensor provides a light tube immersed in the fuel mixture provided to the engine and measures the light transmission of the light tube. The alcohol concentration of the fuel mixture varies the refractive index thereof which, in turn, varies the percentage of light escaping the light tube and thus the light transmissibility thereof. However, aromatics in the in gasoline vary depending on the source of oil from which the gasoline is refined; and the varying aromatics cause a substantial variation in refractive index and thus inaccuracy in the sensor output, since these changes in refractive index do not imply corresponding changes in required A/F ratio. An additional problem with the optical sensor is a clouding of the light path over time, which also affects sensor output.
A capacitive sensor was described in the paper "An On-Board Sensor for Percent Alcohol", published in IEEE Transactions on Vehicular Technology, Vol. VT-27, No. 3, August, 1978. This sensor provided a capacitor with a fuel mixture of varying dielectric constant determining the capacitance thereof. The capacitance was determined by a circuit similar to that described in U.S. Pat. No. 4,001,676 to Hile et al, issued Jan. 4, 1977. The circuit used a DC charge pumping and threshold detection technique in which the capacitor and a reference capacitor were simultaneously charged and allowed to discharge, with the difference in discharge time indicative of capacitance. Unfortunately, the direct current conductivity of a gasoline/methanol mixture increases significantly with methanol concentration. The circuit of the paper was designed for mixtures up to 30 percent methanol and worked adequately up to that level; but increasing methanol concentration beyond that level leads to failure of accurate sensor response as the fuel conductivity essentially shorts out the capacitor.
A microwave sensor is shown in U.S. Pat. No. 4,453,125 to Kimura et al, issued Jun. 5, 1984. This patent shows a microwave oscillator (1-30 GHz) and receiver connected by a tubular dielectric substrate and strip line. The fuel mixture for the engine flows through the tubular dielectric substrate; and the microwaves attenuate by alcohol dielectric loss, whereby the received microwaves indicate alcohol concentration. The extremely high frequencies of the microwave circuit components allow accurate dielectric constant measurement in spite of the conductivity of fuel mixtures containing high alcohol concentrations. However, the microwave components increase the cost of the sensor and generate a high electromagnetic noise (EMI) level.